1. Field of the Invention
The present invention is in the field of bulk particulate solids, and more specifically relates to a testing apparatus and method for determining on the basis of bench-scale testing whether particulate material will flow under the action of gravity through an outlet in the bottom of a container, such as a hopper.
2. The Prior Art
Bulk solids in a divided state such as flour, sugar, ores, powders, dry chemicals, and coal are generally stored in silos that include a hopper at the lower end of the silo through which the bulk solids are to be discharged under the action of gravity. One of the problems of designing such containers is sizing the outlet so that the solids do not form an obstruction by arching across the outlet. The size of the outlet required to prevent arching depends on the physical properties of the bulk solids, specifically, the unconfined yield strength of the material, and the density of the material. The steepness of the walls of the hopper must also be considered.
The present invention is an outgrowth and improvement upon the apparatus shown and described in U.S. Pat. No. 4,719,809, issued Jan. 19, 1988 to Jerry R. Johanson and Kerry D. Johanson. The contents of that patent are hereby incorporated by reference into the present discussion as if reproduced herein verbatim. The patent is believed to be the most relevant prior art in relation to the present invention.
FIG. 1 is adapted from the aforementioned patent and shows a test apparatus described therein. The apparatus includes a mold ring 24' that rests upon a test cell 12'. The test cell 12' includes an inwardly-facing conical surface 14' and the test cell 12' is closed at its lower end by a conical surface 18'. The test cell is filled with a particulate material 32', which is compressed by a weighted disk 26'. After the material 32' has compacted, the mold ring 24' is removed and the material is scraped off even with the top of the test cell 12'.
Next, the entire filled test cell 12' is inverted and a failure load is applied downwardly to the plug 10'. As the failure load is gradually increased, a point is reached at which the material suddenly fails and falls out of the test cell. As discussed at greater length in the aforementioned U.S. Pat. No. 4,719,809, the size of outlet required to prevent arching can be calculated from the compaction load, the failure load, the density of the material, and the shape and dimensions of the outlet. That patent hints, at column 8, lines 21-28, that it might not be necessary to invert the test cell after the consolidation phase, but no apparatus is shown or described for bringing about this result.
In U.S. Pat. No. 3,890,830 issued Jun. 24, 1975 to Dyck, there is shown an apparatus for determining the compressibility and/or moisture content of particulate materials. Although there is a superficial resemblance to the apparatus of the present invention, upon closer study it will be seen that both the apparatus and the method of Dyck's tester are basically different from that of the present invention.
In U.S. Pat. No. 2,633,027 issued Mar. 31, 1953 to Bunnell, there is shown an apparatus and method for testing the flow characteristics of granular materials. The apparatus includes a cylindrical chamber which can selectively be opened and closed at its lower end, and a piston at its upper end for exerting compressive forces on the material. As will be seen below, both the structure and the operation of this apparatus is considerably different from that of the present invention.